Fenitrothion was tested against high density populations of mosquitoes under tropical conditions of wind and temperature. Aerial ULV applications of aqueous solutions were more effective than oil based sprays and the smaller transient droplets not only preclude oil pollution, but also lessen the chances of heavy localized residues of insecticides. With high density mosquitoes, 142 gm/ha gave a near perfect measure of control. Non target insects were affected, however the effect on other aquatic organisms was not significant.